Experimental evidence for the Berg hypothesis: vegetative traits are more sensitive than pollination traits to environmental variation

P>1. In plants with specialized pollination, functionally important floral traits are expected to be under strong selection for accuracy. This may, however, conflict with a general tendency for size-related traits to covary. Previous studies have addressed this fundamental conflict by analysing natural variation across samples of structures, but here we compare the effects of experimentally induced environmental differences on variation in serially homologous pollination and vegetative traits. 2. We examined the effects of experimental variation in nutrient availability and total daily irradiance on two pairs of serially homologous traits in two populations of Dalechampia scandens: (i) The length of the floral bract blade and the length of the leaf blade, and (ii) the length of stipules associated with bracts and leaves. The first pair contrasts a floral trait that is likely to experience canalizing selection (bract blade) with a homologous vegetative trait that does not experience canalizing selection (leaf blade). The second contrasts homologous floral and vegetative traits that are likely to experience similar selective pressures. We also examined variational properties of two blossom traits that interact directly with pollinators: the area of the resin gland and the length of the styles. 3. Variation in the bract blades was decoupled from variation in the vegetative traits and followed the variational patterns of the two blossom traits that are functional in pollination. Stipules associated with bracts and leaves were affected similarly by the experimental treatments in a pattern characteristic of vegetative traits. 4. These results are consistent with the Berg hypothesis of decoupling and canalization of specialized floral structures and support the idea that the variability can evolve in response to selection on variation.